Two-stage melting in cesium lead alloys

The intermediate phase of the intermetallic alloy CsPb is shown to be a plastic crystal characterized by jump reorientations of ${\mathrm{Cs}}_4$ ${\mathrm{Pb}}_4$ structural units. The wave-vector variation of elastic and quasielastic intensities is well reproduced by a simple model of independent structural units jumping between the four orientations observed in the crystal at room temperature. This represents the first observation of a plastic-crystal phase in a metal alloy.